The intended use of a magnetic material, from information storage to power conversion, depends crucially on its domain structure, traditionally crafted during materials synthesis. It is no accident that techniques to mechanically and magnetically harden materials coincide. The same impurities and grain boundaries that prevent dislocations from flowing prevent magnetic domains from switching. Strategies to pin magnetic domains typically are implemented during materials preparation by varying the composition, structure, and morphology, locking in a set of properties that cannot be modified subsequently. Of particular technological interest is ultra-high density magnetic storage, where bits may be warmed close to or above their ferromagnetic transition temperature for writing and then cooled for long-term retention and reading. Although such heat-assisted magnetic recording approaches have enabled improvements in storage density, the technologies introduce new complexities and thermodynamic challenges, such as preserving the states of bits adjacent to those being written.